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Thcre are marcs wńh multiple endomctrial cysts I wbkh can make the search and detection uf twin ves-J śfes in the uterus partieulariy difficult (Fig. 1.75). Oc-( caaooaily it is difficult to fmd the membranę that di* vides the two vesfcies of a twin pregnancy (Fig. 1.76).

| Where uncertainty exits the finał dectston should be 1 postponed until the time when the detection of two beart beats can be expected.

The indniduai embryonic yesides of a normal twin pregnancy often show the same growth ratę as that seen in singlewn pregnancies (Ginther 1984 b, Pipers et al. 1984).

In spite of that the sizes of individual vesicles in cases of twin pregnancies can differ (Fig. 1.77). This may in-tficate that the smaller member of a twin is retarded and is in the process of undergoing an embryonic death. However, in cases where twin yesicles of uneąual size are detected the categorical conclusion that the smaller embryo must be dying, will often prove wrong. Asyn-chronous double ovulatioas with an interval of 48 to 96 hours between ovuIations can lead to twin pregnancies. The two conceptuses will show differences in their stages of deveiopment, but can both continue to persist and grow into the fetal stage of pregnancy.

In nmlateraOy fixed adjacent twins an embryonic mortality is Bkely between Days 17 and 29 (Ginther 1984 c). Prior to this, during the phase of mobility, re-soiptions are rare and after this until Day 45 only few mortalities will occur. The resorption of one member of a twin is morę likely than that of both. Bilaterally fixed twins continue to deve!op in the majority of cases and

lead to abortkm during the advanccd stages of pregnancy.

The following approach is recommended for vct-erinary piactice: Where twins of clearly differing sizcs are diagnosed until Day 16 of gestation, the possibility of a spontaneous resorption should be coasidcrcd. In* tereention may be postponed until 2 to 3 days latcr when it is evident Erom the follow-up cxamination that both embryos have continued to devclop sińce the pre-vious esamination. Where twins are found before Day 16 and both vesicles appear to be intact it is usefiil to attempt the reduction of the pregnancy to a singleton pregnancy by manually crushing of either of the two ves-icles. Since both vesicles are still mobile at this stage, repeated examinations can be performed until the two vesicles are found to be far enough apart so that the one can be crushed without harming the second one. Lf a bilaterally fixed twin pregnancy is first detected after Day 16 an intervention is indicated, because it is likely that both embryos will persist. Since a spontaneous resorption is morę likely in unilaterally fixed twins after Day 16 an intervention can be postponed for some time.

Once a twin pregnancy has been reduced by crushing of one vesicle the successful elimination of the one and the further developrnent of the other vesicle should be monitored ultrasonically. Where an embryo has been eliminated by crushing it before Day 20 the survival chance for the second conceptus is good, but in later cases one must expect the second embryo to die (Bowman 1986).